A disk drive is a digital data storage device that stores information on concentric tracks on a storage disk. The storage disk is coated on one or both of its primary surfaces with a magnetic material that is capable of changing its magnetic orientation in response to an applied magnetic field. During operation of a disk drive, the disk is rotated about a central axis at a constant rate. To read data from or write data to the disk, a magnetic transducer (or head) is positioned above (or below) a desired track of the disk while the disk is spinning.
Writing is performed by delivering a polarity-switching write current signal to the magnetic transducer while the transducer is positioned above (or below) the desired track. The write signal creates a variable magnetic field at a gap portion of the magnetic transducer that induces magnetically polarized transitions on the desired track. The magnetically polarized transitions are representative of the data being stored.
Reading is performed by sensing the magnetically polarized transitions on a track with the magnetic transducer. As the disk spins below (or above) the transducer, the magnetically polarized transitions on the track induce a varying magnetic field into the transducer. The transducer converts the varying magnetic field into a read signal that is delivered to a preamplifier and then to a read channel for appropriate processing. The read channel converts the read signal into a digital signal that is processed and then provided by a controller to a host computer system.
When data is to be written to or read from the disk, the transducer must be moved radially relative to the disk to a desired track. In a seek mode, the transducer is moved radially inwardly or outwardly to arrange the transducer above the desired track. In an on-track mode, the transducer reads data from or writes data to the desired track. The tracks are typically not completely circular. Accordingly, in the on-track mode the transducer must be moved radially inwardly and outwardly to ensure that the transducer is in a proper position relative to the desired track. The movement of the transducer in on-track mode is referred to as track following.
Modern hard disk drives may employ a dual-actuator system for moving the transducer radially relative to the disk. A first stage of a dual-actuator system is optimized for moving the transducer relatively large distances. A second stage of a dual-actuator system is optimized for moving the transducer relatively small distances. The present invention relates to hard disk drives having dual-stage actuator systems.
FIG. 1 depicts a disk drive 10 comprising control electronics typically including a preamplifier, a read/write channel, a servo control unit, a random access memory (RAM), and read only memory (ROM), spindle motor and VCM controller driving electronics. The preamplifier, read/write channel, servo control unit, RAM, and ROM are or may be conventional and will not be described herein beyond what is necessary for a complete understanding of the present invention.
FIG. 1 shows that the disk drive 10 includes a disk 12, a spin motor 14, and a base plate 16. The disk 12 is rotated by a spin motor 14, and the spin motor 14 is mounted to a base plate 16. The disk drive 10 includes at least one and typically a plurality of disks 12, each with one or two recording surfaces. During use, the disk 12 is rotated about a spindle axis A shown in FIG. 2.
The disk drive 10 further comprises what is commonly referred to as a head 18. The head 18 comprises or supports the magnetic read/write transducer described above and will thus be referred to herein as the component of the disk drive 10 that reads data from and writes data to the disk 12.
FIGS. 1 and 2 further illustrate a positioning system 20 of the disk drive 10. The positioning system 20 comprises a bearing assembly 22 that supports at least one actuator arm assembly 24. The actuator arm assembly 24 supports the head 18 adjacent to one recording surface 26 of one of the disks 12. Typically, the bearing assembly 22 will support one actuator arm assembly 24 and associated head 18 adjacent to each of the recording surfaces 26 of each of the disks 12. The actuator arm assemblies 24 allow each head 18 to be moved as necessary to seek to a desired track 28 in seek mode and then follow the desired track 28 in track following mode.
The exemplary positioning system 20 depicted in FIGS. 1 and 2 is a dual-stage system. Accordingly, each actuator arm assembly 24 comprises a first actuator 30 and a second actuator 32. The principles of the present invention are currently of primary importance when applied to the second actuator of a dual-stage actuator system, and that application of the present invention will be described herein.
For ease of illustration, FIGS. 1 and 2 depict the first and second actuators 30 and 32 as comprising elongate arms 34 and 36, respectively, and the actuators 30 and 32 may be implemented as shown in FIGS. 2 and 3. Conventionally, the bearing assembly 22 is also considered part of the first actuator 30.
As perhaps best shown in FIG. 2, the bearing assembly 22 supports a proximal end 40 of the arm 34 of the first actuator 30 for rotation about a first axis B, while a distal end 42 of the first actuator arm 34 supports a proximal end 44 of an arm 36 of the second actuator 32 for rotation about a second axis C. In this case, the head 18 is supported on a distal end 46 of the second actuator arm 36.
The actuators 30 and 32 may, however, be implemented using other structures or combinations of structures. For example, the first actuator 30 may comprise an elongate arm that rotates about a first axis B, while the second actuator 32 may comprise a suspension assembly rigidly connected to a distal end of the first actuator. In this case, the first actuator is able to rotate about an actuator axis, while the head 18 would be suspended from the second actuator for linear movement along the disk radius relative to the position of the first actuator. The actuators 30 and 32 may thus take any number of physical forms, and the scope of the present invention should not be limited to the exemplary actuators 30 and 32 depicted in FIGS. 2 and 3 and described herein.
FIG. 2 also illustrates that the exemplary actuators 30 and 32 of the positioning system 20 further comprise a first electromechanical transducer 50 and a second electromechanical transducer 52. In response to a first control signal, the first transducer 50 moves the first actuator arm 34 to change an angular position of the head 18 relative to the first axis B. The second transducer 52 is supported by the distal end 42 of the first actuator 30 to rotate the head 18 about the second axis C in response to a second control signal. The first transducer 50 may be a voice coil motor (VCM), while the second transducer 52 may be a piezo-electric transducer (PZT), but other types of transducers may be used as the first and second transducers 50 and 52.
In FIG. 2, an angular position of the first actuator arm 34 relative to the first axis B is represented by reference character D, while an angular position of the second actuator arm 36 relative to the second axis C is represented by reference character E. When the head 18 is above the neutral position D (on-track mode), the displacement of the second actuator arm 36 is zero.
FIG. 2 also shows that a range of movement utotal is associated with the second actuator 36 relative to a neutral position D defined by the first actuator arm 34. The stroke of the actuator arm 36 in either direction from the neutral position will be referred to herein as umax. The terms u+max and u−max used in FIG. 2 indicate the direction of the stroke with respect to the neutral position D.
An actual position uA of the second stage actuator 32 corresponds to the angular position of the second actuator 32 relative to the neutral position D at any point in time. An initial offset uO of the second stage actuator 32 is the actual position signal uA of the second stage actuator 32 at the time a seek operation is initiated. The terms uO+ and uO− will be used to identify not only the magnitude but also the direction of the initial offset.
FIG. 2 further identifies arbitrary first and second tracks 28a and 28b on the disk 12. The actuator arm assembly 24 is shown in an initial position by solid lines and in a target position by broken lines; the first track 28a will thus be referred to as the “initial track” and the second track 28b will be referred to as the “target track”. It should be understood that the terms “initial track” and “target track” are relative to the position of the head 18 before and after a seek operation. Any track 28 on the disk 12 may be considered the initial track or the target track depending upon the state of the disk drive 10 before and after a particular seek operation.
FIG. 3 contains a block diagram of a servo system 60 incorporating a conventional two-stage actuator system. The servo system 60 will typically be embodied as a software program running on a digital signal processor, but one of ordinary skill in the art will recognize that control systems such as the servo system 60 described herein could be implemented in other forms such as using discrete hardware components.
The servo system 60 comprises a first stage servo 62 and a second stage servo 64. As described above, the disk 12 defines a plurality of tracks 28 in the form of generally concentric circles centered about a spindle axis C. The first stage servo 62 controls the first transducer 50 and the second stage servo 64 controls the second transducer 52 to support the head 18 adjacent to a desired one of the tracks 28. The first and second actuator control signals are generated as part of this larger servo system 60.
More specifically, an input signal R is combined with a position error signal PES by a first summer 70. The second stage position signal Y2 is indicative of an actual position signal uA of the transducer 52 of the second stage servo 64, and a second stage position estimate signal Y2est is indicative of an estimated position of the transducer 52 of the second stage servo 64. The second summer 72 combines the second stage position estimate signal Y2est and the output of the first summer 70. A first stage position signal Y1 is indicative of the actual position signal uA of the first transducer 50 of the first stage servo 62. A third summer 74 combines the first and second stage position signals Y1 and Y2. System disturbances d are represented as an input to the third summer 74. The position error signal PES thus represents the combination of the first and second position signals Y1 and Y2 with any system disturbances d.
The sources of the input signal R and the first and second stage position signals Y1 and Y2 are or may be conventional and will be described herein only to the extent necessary for a complete understanding of the present invention. Briefly, each of the tracks 28 includes data sectors having stored data and servo sectors having servo data. The servo data identifies each individual track 28 to assist in seek operations and is also configured to allow adjustment of the radial position of the head 18 during track following. A servo demodulation unit generates the position error signal PES and the first and second stage position signals Y1 and Y2 based on the servo data read from the disk 12. The input signal R is generated by a host computer or is simply zero during track following.
The seek operation may be divided into a seek phase and a settle phase. During the seek phase, the servo system 60 displaces the head 18 most of the distance from the initial track 28a to the target track 28b. 
During the settle phase, the second stage servo 64 is conventionally deactivated, and the head 18 repeatedly crosses over the target track 28b as the relatively low bandwidth first stage servo 62 of the servo system 60 attempts to lock onto the desired track 28b. The second stage servo 64 is typically deactivated while the disk drive 10 performs a seek operation because the stroke S of the second stage servo 64 is too limited to have any significant effect during the seek phase of a seek operation.
Once the seek portion of the seek operation is completed, however, the relatively high bandwidth second stage servo 64 would ideally be activated to speed up the settle phase of the seek operation.
The fundamental problem with using the second stage servo 64 to assist during the settle phase of the seek operation is when to deactivate and reactivate the second stage actuator. The conventional method is to set the actuator 64 to the neutral position at the beginning of the seek operation, deactivate the actuator 64, and then reactivate the actuator 64 when the position error signal PES equals zero.
The conventional method of improving settle times using the second stage servo 64 creates several problems. First, the first stage servo 62 takes too long to lock onto the desired track to place the system 10 in track following mode. The second stage servo 64 would ideally be activated earlier in the seek operation to assist the actuator system 60 in finding the desired track. Second, the second stage servo 64 may be significantly offset from the neutral position (out of stroke) when it is determined that the system is on-track. In this case, the second stage servo 64 may saturate and will thus be unable to cancel out displacement of the first stage servo 62, which may not yet have settled. As a result of this post-settle saturation, a bump in the position error signal PES may occur.
The need thus exists for improved systems and methods of controlling the second stage of a dual stage actuator during the settle phase of a seek operation.